This invention relates generally to vehicle electronic components which communicate in the electromagnetic spectrum. More particularly, this invention relates to a single vehicle antenna system which receives signals for a plurality of electronic components which operate in different spaced apart frequency bands.
Vehicles typically include a multiple of various electronic components. Each of the components typically operates in different spaced apart frequency bands. Examples of such electronic components include an FM radio, which operates at approximately 100 MHz, and remote keyless entry (RKE) systems, which operate at approximately 315 MHz. Each component must therefore include its own frequency specific antenna system.
Known antenna systems may include a diplexer which allows a single antenna to receive a different frequency for each associated vehicle electronic component. The diplexer typically has at least two branches. Each branch includes a band pass filter for passing signals in a portion of the frequency spectrum different from the portions passed by the other branches. Each component thereby receives only its particular operating signal. Each diplexer branch band pass filter passes only its particular signal while blocking all other signals. Each band pass filter also tends to attenuate the signals for the other branches. A transmitter of a higher power output or a more sensitive receiver must therefore be used for each vehicle component. The expense involved in increasing vehicle electrical components receiver sensitivity typically outweighs the benefits of providing a single antenna system.
Accordingly, it is desirable to combine a single antenna system with a plurality of vehicle components while minimizing the attenuation of the passed signals in one branch by the filter in another branch.
In general terms, this invention includes an antenna system connected to at least two vehicle electronic components which communicate through signals in spaced apart frequency bands. In one disclosed embodiment the first component is associated with a resonant frequency of an FM radio, which operates at approximately 100 MHz, and the second component is associated with a resonant frequency of an Remote Keyless Entry system (RKE), which operates at approximately 315 MHz.
The antenna is coupled to a diplexer at a common branch. The radio is connected to a first equipment branch of the diplexer by a first transmission line and the RKE system is connected to a second equipment branch of the diplexer unit by a second transmission line. In a preferred embodiment, the first transmission line is of a length equal to one quarter the wavelength of the second signal (xcex/4 at 315 MHz), while the second transmission line is of a length equal to one quarter the wavelength of the first signal (xcex/4 at 100 MHz).
The first transmission line which carries the 100 MHz first signal is matched to be a short circuit at 315 MHz and the second transmission line which carries the 315 MHz second signal is matched to be a short circuit at 100 MHz. At the diplexer, the 100 MHz signal of the first signal, is not affected by the 315 MHz signal of the second signal as the 315 MHz signal appears as an open circuit because of the xcex/4 at 315 MHz length of the first transmission line. Conversely, the 315 MHz signal of the second signal, is not affected by the 100 MHz signal of the first signal as the 100 MHz signal appears as an open circuit because of the xcex/4 at 100 MHz length of the second transmission line.
Accordingly, by relating the length of the first and second transmission lines, each operates as a short in relation to the signal carried by the other which substantially eliminates attenuation. Thus, instead of the passed signals being attenuated, only the passed signal is xe2x80x9cseenxe2x80x9d by the junction.